Thermoplastic compositions having high dimensional stability

ABSTRACT

Thermoplastic compositions comprising a bulk polymer, an additive, and a compatibilizer/emulsifier/surfactant (CES) are described. The bulk polymer comprises an alkylene terephthalate or naphthalate polyester such as polyethylene terephthalate. The additive comprises an amorphous or substantially amorphous co-polymer of ethylene and a co-monomer that forms polar portions, such as methylacrylate, butylacrylate, ethylacrylate, or ethylhexyl methacrylate. The CES comprises a co-polymer or terpolymer of ethylene and a glycidyl acrylate or maleic anhydride, and optionally an acrylate such as methylacrylate, ethylacrylate, propylacrylate, butylacrylate, ethylhexyl acrylate, or mixtures thereof. The thermoplastic compositions of the present invention exhibit improved molding properties, high dimensional stability, high temperature resistance, and are particularly useful in food-grade applications such as dual-ovenable containers.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of U.S. application Ser. No.09/453,457, filed Dec. 2, 1999, the disclosure of which is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention is directed to polymeric materials and,more particularly, to thermoplastic polyester compositions having highdimensional stability at elevated temperatures with improved toughness,which especially are useful in food-grade applications such asdual-ovenable containers.

BACKGROUND OF THE INVENTION

[0003] Polyesters are polymeric materials typically made by acondensation reaction of dibasic acids and dihydric alcohols. Commonexamples of polyesters include alkylene terephthalate and naphthalatepolymers such as polyethylene terephthalate (PET), polybutyleneterephthalate (PBT), polyethylene naphthalate (PEN),polycycloterephthate (PCT), polycycloterephthatlic acid (PCTA),(poly)ethylene-co-1,4-cyclohexane-dimethylene terephthalate (PETG), andpolytrimethylene terephthalate (PTT). Polyethylene terephthalate (PET)and polybutylene terephthalate (PBT) are examples of polyesters havingexcellent barrier properties, excellent chemical resistance, gooddimensional stability at room temperature, and high abrasion resistance.However, polyalkylene terephthalates tend to become brittle uponcrystallization, especially upon thermal crystallization, and are notdimensionally stable at temperatures above their glass transitiontemperature (T_(d). As a consequence, non-oriented, thermallycrystallized polyalkylene terephthalates have poor ductility, poorimpact resistance, and poor heat resistance, which limit the utility ofthe polymer in many applications. Several attempts have been made toimprove the impact properties of polyalkylene terephthalates, includingthe addition of various impact modifiers as described in U.S. Pat. Nos.4,172,859, 4,284,540, and 4,753,980. U.S. Pat. No. 4,753,980 disclosesthe use of an ethylene/n-butylene acrylate/glycidyl methacrylateter-polymer to produce toughened polyester.

[0004] Another class of polymer having widespread utility ispolyethylenes, which are ethylene-based polyolefin polymers.Polyethylenes most often are linear but also can be branched. Linearpolyethylenes typically are classified by density, e.g., low-densitypolyethylene (LDPE), medium-density polyethylene (MDPE), high-densitypolyethylene (HDPE), and the like. Polyethylenes exhibit good toughness,low moisture absorption, high chemical resistance, excellent electricalinsulating properties, low coefficient of friction, and ease ofprocessing. However, polyethylenes have poor load-bearing and gasbarrier characteristics, and relatively poor heat resistance properties.

[0005] Numerous attempts have been made to combine polyalkyleneterephthalates and polyethylene polymers in efforts to realize theabove-described useful properties of each of the two types of polymer.However, polyalkylene terephthalates and polyethylenes are highlyincompatible because of their significantly different physical andchemical properties, such as solubility, surface tension, and polarity.Combining the two polymers typically results in a phase-separated blendexhibiting poor mechanical properties, especially impact properties. Inaddition, each of the polymers retains its own thermal properties.Therefore, the phase-separated blend thermally degrades at the lowerdegradation temperature of the two polymers. Such a blend is entirelyunsatisfactory.

[0006] Various thermoplastic elastomers have been proposed to improvethe compatibility of polyalkylene terephthalates and polyethylenes.Traugott et al., J Appl. Poly. Sci., 28, 2947 (1983), describes blendsof polyethylene terephthalate and high density polyethylene (PET/HDPE)which are said to exhibit high ductility. The blend compositions utilizeup to 20 weight percent of a styrene/ethylene-butadiene/styrenetri-block co-polymer (SEBS) or an ethylene-propylene co-polymer as acompatibilizing thermoplastic elastomer.

[0007] The relatively large concentrations of the thermoplasticelastomers which are required to improve compatibility, however, candiminish other desirable properties of the polymer blends, most notablyimpact properties and thermal stabilities. U.S. Pat. No. 5,436,296describes a co-polymer of a C₂-C₁₀ alpha-olefin and a glycidyl orisocyanate group-containing functional compound which is said tocompatibilize thermoplastic blends of polyalkylene terephthalates andpolyethylenes while improving impact properties and heat resistance. Thethermoplastic blend is described as a continuous matrix of polyalkyleneterephthalate with polyethylene domains dispersed therein.

[0008] Food grade containers which can be used for cooking orreconstituting foodstuffs are of particular interest. Such containers,whether disposable or intended for re-use, typically are heated totemperatures exceeding 250° F. and must be capable of being heated to atleast about 350-400° F. without significant distortion of the rigidpackage if the container is to be considered ovenable. Food containersmade of polymeric materials are used in a wide variety of applications.For example, foamed polystyrene is widely used in making hot drink cups.It is also used in making “clam shells” which are used by the fast foodindustry as packages for hamburgers and other types of sandwiches. Onedrawback associated with the use of polystyrene is the possiblemigration of residual styrene into food products, especially when thecontainer is reheated, e.g., by a microwave oven. There are strictlimitations on the quantities of styrene and various other plasticscomponents which may be liberated from a plastic container into food inthe container.

[0009] The wide spread popularity of microwave ovens for home use hasinitiated interest in food trays which can be used in either microwaveovens or convection ovens. Such trays are of particular value ascontainers for frozen prepared foods. It is important for such trays tohave good impact strength and dimensional stability at both freezer andoven temperatures. Of course, it also is important for such trays to becapable of withstanding rapid heating from freezer temperatures of about−22° F. to oven temperatures exceeding about 250° F.

[0010] Containers which are capable of being heated in either convectionovens or microwave ovens are sometimes described as being dual-ovenable.Polyesters are highly suitable for use in making such dual-ovenablecontainers. However, it is important for the polyester to be in thecrystalline state rather than the amorphous state in order to achievesatisfactory high temperature stability. Normally, polyesters willundergo crystallization by heat treatment at elevated temperatures andthe crystallites formed will remain substantially stable up to near themelting point of the polyester. As a general rule, dual-ovenablecontainers which are comprised of polyester will be heat-treated toattain a crystallinity of higher than about 20%.

[0011] Injection molding and thermoforming are widely known methods forforming thermoplastic polyester articles. In injection molding, thepolyester is heated above its melting point and injected undersufficient pressure to force the molten polyester to fill the moldcavity. The molten polyester is cooled in the mold until it is rigidenough to be removed. Injection molding of a polyester compositioncontaining 0.5% to 10% by weight isotactic polybutene-1 is described inU.S. Pat. No. 3,839,499. This injection molding method, however,generally is not satisfactory for the production of thin walledarticles, such as dual-ovenable trays, due to flow lines and layeringwhich develop during the filling of the mold which lead to non-uniformproperties, surface irregularities, and warping of the finished article.

[0012] Thermoforming is another process which is used commercially inthe production of polyester articles. It is a particularly valuabletechnique for use in producing thin walled articles, such asdual-ovenable food trays, on a commercial basis. In thermoforming, asheet of preformed polyester is preheated to a temperature sufficient toallow deformation of the sheet. The sheet is then made to conform to thecontours of a mold by such means as vacuum assist, air pressure assist,or matched mold assist. The thermoformed article produced is normallyheat treated in the mold in order to attain a crystallinity of at leastabout 20%.

[0013] Crystallization rates generally can be improved by including asmall amount of a nucleating agent in polyester compositions. Forexample, U.S. Pat. No. 3,960,807 describes a process for thermoformingarticles from a polyester composition having an intrinsic viscosity(I.V.) of at least 0.75 which is comprised of (1) a crystallizablepolyester, (2) a crack stopping agent, preferably a polyolefin, and (3)a nucleating agent. Polyester articles which are made utilizing suchcompositions generally have improved mold release characteristics andimproved impact strength. Additionally, the utilization of such modifiedpolyester compositions results in faster thermoforming cycle times dueto the associated faster rate of crystallization. U.S. Pat. No.4,981,631 describes thermoforming a substantially amorphous cellularsheet which is comprised of (a) from about 94 to 99 wt % polyethyleneterephthalate having an I.V. of at least 0.7 dl/g, (b) from about 1 to 6wt % of at least one polyolefin, and (c) a sufficient amount of inertgas cells to provide the cellular sheet with a density within the rangeof about 0.4 to 1.25. Thermoforming is said to be carried out in aheated mold for a time sufficient to achieve a crystallinity of fromabout 5% to 45%.

[0014] U.S. Pat. No. 5,409,967 describes an amorphous, aromaticpolyester such as PET having an initial I.V. of at least 0.7 dl/gblended with an impact modifier. The impact modifier is described as acore-shell polymer with cores comprised mainly of rubbery polymers ofdiolefins and vinyl aromatic monomers, and shells comprised mainly ofstyrene co-polymers such as styrene and hydroxyalkyl (meth)acrylate. Theimpact modifier is said to substantially increase impact strength ofamorphous, aromatic polyesters without detracting from clarity(transparency).

[0015] Polyesters presently used in dual-ovenable trays generally arerequired to have an I.V. of at least 0.95 dl/g in order for the heat-setarticles to be food-grade, to have sufficient impact strength at lowtemperatures (e.g., as in a freezer), and to have sufficient toughnessand dimensional stability over a broad temperature range.

[0016] It would be desirable to develop a polyester composition havingimproved molding properties, especially one which is a thermoplasticcomposition having high dimensional stability, and high temperatureresistance, which is thermally stable (e.g., capable of beingreprocessed or recycled without loss of toughness or generation ofdegradation by-products), and which is useful in food-grade applicationssuch as in making dual-ovenable containers. It would be especiallydesirable to develop a toughener additive which permits the use ofpolyesters having lower I.V. (e.g., less than 0.95) in heat-set articleswhich are food-grade, which have high dimensional stability and hightemperature resistance, which are thermally stable, and whichadditionally retain toughness, especially at low temperatures.

BRIEF SUMMARY OF THE INVENTION

[0017] According to one embodiment, the present invention is directed toa polyester thermoplastic composition comprising a bulk polymer, anadditive in a concentration from about 4 wt % to about 40 wt %, and acompatibilizer/emulsifier/surfactant (CES) in a concentration from about0.1 wt % to about 8 wt %, based on the total weight of the composition.The bulk polymer comprises an alkylene terephthalate or naphthalatepolyester such as polyethylene terephthalate (PET). The additivecomprises an amorphous or substantially amorphous co-polymer of ethyleneand at least one co-monomer that forms polar portions, such as anacrylate, e.g., methylacrylate, butylacrylate, ethyl acrylate,ethylhexyl methacrylate, or a mixture thereof. The CES comprises agrafted or backbone co-polymer or ter-polymer of ethylene and a glycidylacrylate or maleic anhydride, and optionally methylacrylate,butylacrylate, ethyl acrylate, ethylhexyl methacrylate, or mixturesthereof.

[0018] According to another embodiment of the present invention, anadditive for providing toughness to a thermoplastic composition consistsessentially of a substantially amorphous ethylene co-polymer with anacrylate co-monomer concentration of from about 7 wt % to about 40 wt %,based on the total weight of the co-polymer, and optionally a core-shelltoughener.

[0019] According to another embodiment of the present invention, acontainer comprises a molded thermoplastic composition which can be heatset and which includes a bulk polymer, an additive, and acompatibilizer/emulsifier/surfactant (CES). The container can containone or more solid layers, cellular layers, or a combination thereof.

[0020] According to yet another embodiment of the invention, afood-grade cooking container can be made by thermal crystallization of anon-oriented or substantially non-oriented composition comprising a bulkpolymer of an alkylene terephthalate or naphthalate having an intrinsicviscosity of less than 0.95, preferably less than about 0.90, morepreferably less than about 0.85, and even more preferably less thanabout 0.80, wherein the bulk polymer is present in a concentration of atleast about 30 wt %, preferably at least about 45 wt %, more preferablyat least about 55 wt %, and even more preferably at least about 60 wt %,based on a total weight of the composition. The food-grade container cancontain one or more solid layers, cellular layers, or a combinationthereof.

[0021] According to a further embodiment of the invention, a method ofimparting elasticity to an alkylene terephthalate or naphthalatepolyester comprises adding to the bulk polymer an effective amount of asubstantially amorphous ethylene co-polymer with an acrylate co-monomerconcentration of from about 7 wt % to about 40 wt %, based on a totalweight of the ethylene co-polymer. Suitable effective amounts of theethylene co-polymer typically range from about 4 wt % to about 40 wt %,based on the total weight of the bulk polymer and ethylene co-polymer.

[0022] According to a further embodiment of the invention, a method ofimproving processibility of an alkylene terephthalate or naphthalatepolyester comprises adding to the bulk polymer an effective amount of asubstantially amorphous ethylene co-polymer with an acrylate co-monomerconcentration of from about 7 wt % to about 40 wt %, based on a totalweight of the ethylene co-polymer. Suitable effective amounts of theethylene co-polymer typically range from about 4 wt % to about 40 wt %,based on the total weight of the bulk polymer and ethylene co-polymer.

[0023] According to another embodiment of the invention, a food-gradethermoplastic composition comprises a bulk polymer, an additive, and acompatibilizer/emulsifier/surfactant (CES).

[0024] According to yet another embodiment of the invention, afood-grade cooking container comprises a bulk polymer, an additive, anda compatibilizer/emulsifier/surfactant (CES). The food-grade containercan contain one or more solid layers, cellular layers, or a combinationthereof.

[0025] According to a further embodiment of the invention, acompatibilizer/emulsifier/surfactant (CES) for use with polyestercompositions is selected from ethylene/maleic anhydride co-polymer,ethylene/glycidyl methacrylate/ethylhexyl acrylate ter-polymer,ethylene/maleic anhydride/methylacrylate ter-polymer, ethylene/maleicanhydride/ethylacrylate ter-polymer, ethylene/maleicanhydride/butylacrylate ter-polymer, ethylene/maleicanhydride/ethylhexylacrylate ter-polymer, and mixtures thereof.

[0026] Preferred thermoplastic compositions of the present inventionexhibit properties which are especially desirable in food-gradeapplications. One such property is a relatively low to medium I.V. ofPET, which improves extrusion and molding of a heat-set product withimproved toughness. Other such properties include high dimensionalstability, high temperature resistance, toughness, processibility, andimproved molding detail. Further, the composition is thermally stable,resulting in low extractives and less degradation and ensuringfood-grade compliance. Such properties render the thermoplasticcomposition of the present invention especially suitable for use indual-ovenable containers, which require high dimensional stability andsealability for high-speed packaging and distribution of frozen orrefrigerated food products. The ability to use lower-I.V. polyesterspermits more economical production of heat-set products having highdimensional stability and resistance to degradation over broadtemperature ranges.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The thermoplastic composition of the present invention comprisesa bulk polymer, an additive, and a compatibilizer/emulsifier/surfactant(CES). The thermoplastic composition can have high dimensionalstability, high temperature resistance, and is particularly useful infood-grade applications such as in the manufacture of ovenable (e.g.,conventional, convection, and microwave) containers. Preferably, thepolyester thermoplastic composition is not oriented prior tothermoforming to provide heat-set containers having dimensionalstability at elevated temperatures.

[0028] Unless otherwise indicated, all percentages set forth herein areweight percentages based on the total weight of the thermoplasticcomposition.

[0029] As used herein alone or as part of another group, the term“alkyl” or “alk” denotes straight and branched chain saturatedhydrocarbon groups, preferably having 1 to 20 carbon atoms, more usually1 to 6 carbon atoms. Exemplary groups include methyl, ethyl, propyl,isopropyl, n-butyl, t-butyl, isobutyl, pentyl, hexyl, isohexyl, heptyl,4,4-dimethylpentyl, octyl, 2,2,4-trimethylpentyl, nonyl, decyl, undecyl,dodecyl, combinations thereof and the like.

[0030] The term “cycloalkyl” as used herein alone or as part of anothergroup, denotes saturated cyclic hydrocarbon ring systems, preferablycontaining 1 to 3 rings and 3 to 7 carbons per ring. Exemplary groupsinclude cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl,cyclooctyl, cyclodecyl, cyclododecyl, adamantyl and combinationsthereof.

[0031] The term “alkylene” as used herein denotes divalent, unsaturatedhydrocarbon groups of the formula —C_(n)H_(2n)—, wherein n preferably isfrom 1 to 10. Exemplary groups include methylene, ethylene, andpropylene. Such groups represent alkyl groups as defined above fromwhich another hydrogen has been removed.

[0032] Intrinsic viscosity (I.V.) as used herein is defined as the limitof the fraction In (v)/C as C, the concentration of the polymersolution, approaches 0, wherein v is the relative viscosity which ismeasured at several different concentrations in a 60/40 mixed solvent ofphenol and tetrachloroethane at 30° C. Units for I.V. are dl/g unlessotherwise indicated.

[0033] The bulk polymer of the thermoplastic composition comprises analkylene terephthalate or naphthalate polyester. Polyalkyleneterephthalates can be prepared by the polycondensation reaction ofterephthalic acid, or a lower alkyl ester thereof, and aliphatic orcycloaliphatic C₂-C₁₀ diols. Such reaction products include polyalkyleneterephthalate resins, including, but not limited to, polyethyleneterephthalate, polybutylene terephthalate, polytetramethyleneterephthalate, and copolymers and mixtures thereof. As is known to theart, these polyester resins may be obtained through the polycondensationreaction of terephthalic acid, or a lower alkyl ester thereof, and analkylene diol. For example, polyethylene terephthalate can be preparedby polycondensation of dimethyl terephthalate and ethylene glycolfollowing an ester interchange reaction. Non-limiting examples ofsuitable polyesters include polyethylene terephthalate (PET),polybutylene terephthalate (PBT), polyethylene naphthalate (PEN),polycycloterephthalate (PCT), polycycloterephthatlic acid (PCTA),(poly)ethylene-co-1,4-cyclohexanedimethylene terephthalate (PETG),polytrimethylene terephthalate (PTT), and co-polymers and mixturesthereof.

[0034] The bulk polymer may be a homopolymer, co-polymer, or blendsthereof, and may be straight-chained, branched, or mixtures thereof. Inaddition, blends of polymers having varying molecular weights and/orintrinsic viscosity (I.V.) may be used. Typically, I.V. ranges fromabout 0.5 to 1.2. The polymer may be branched by inclusion of smallquantities of trihydric or tetrahydric alcohols, or tribasic ortetrabasic carboxylic acids, examples of which include trimellitic acid,trimethylol-ethane, trimethylolpropane, trimesic acid, pentaerythritoland mixtures thereof. The degree of branching preferably is no more thanabout 3%. The bulk polymer may comprise, in whole or in part, recycledpolyesters.

[0035] The bulk polymer may contain up to about 25 mol % of otheraliphatic dicarboxylic acid groups having from about 4 to about 12carbon atoms as well as aromatic or cycloaliphatic dicarboxylic acidgroups having from about 8 to about 14 carbon atoms. Non-limitingexamples of these monomers include iso-phthalic acid (IPA), phthalicacid, succinic acid, adipic acid, sebacic acid, azelaic acid,cyclohexane diacetic acid, naphthalene-2,6-dicarboxylic acid,4,4-diphenylene-dicarboxylic acid and mixtures thereof.

[0036] The bulk polymer also may contain up to about 25 mol % of otheraliphatic C₂-C₁₀ or cycloaliphatic C₆-C₂₁ diol components. Non-limitingexamples include neopentyl glycol, pentane-1,5-diol,cyclohexane-1,6-diol, cyclohexane-1,4-dimethanol, 3-methylpentane-2,4-diol, 2-methyl pentane-2,4-diol, propane-1,3-diol, 2-ethylpropane-1,2-diol, 2,2,4-trimethyl pentane-1,3-diol, 2,2,4-trimethylpentane-1,6-diol, 2,2-dimethyl propane-1,3-diol, 2-ethylhexane-1,3-diol, hexane-2,5-diol, 1,4-di(β-hydroxyethoxy)benzene,2,2-bis-(4-hydroxy-propoxyphenyl)propane, and mixtures thereof.

[0037] Linear alkylene terephthalate or naphthalate homopolymerstypically exhibit faster crystallization than do co-polymers. Branchedpolymers typically yield higher melt strengths. As will be appreciatedby those skilled in the art, mixtures of branched- or unbranchedhomopolymers and/or co-polymers, optionally having varying molecularweights and/or I.V., can be selected to obtain a polymer having the mostsuitable properties for a particular application.

[0038] Polymers having lower I.V. generally have lower molecularweights, shorter chain lengths, and exhibit faster crystallizationkinetics, resulting in better heat setting properties (e.g., higherdimensional stability). In addition, lower-I.V. polymers generally areless expensive, and have lower extrusion melt temperatures, resulting inless degradation, faster stress relaxation time, reduced molding timeand reduced production time. However, such polymers traditionally areconsidered unsuitable for making ovenable containers because of poortoughness, low melt strength, poor web handling characteristics, andpoor ductility. The present invention advantageously overcomes thedrawbacks conventionally associated with the use of lower-I.V. polymers,thereby permitting their use in food-grade, heat-set products.

[0039] The bulk polymer of the present invention may contain variousimpurities. Preferably, impurities that hinder crystallization are heldto a minimum. Examples of such impurities include acetylaldehyde,diethylene glycol, and isopropyl aldehyde, with preferred maximumconcentrations of these components being 2 wt %, 2 ppm, and 5 wt %,respectively, based on the total weight of the bulk polymer. Skilledpractitioners can easily identify the impurities that hindercrystallization and the concentration at which they do so. Otheradditives known in the art may be included in the composition up toabout 30% by weight. Non-limiting examples of such additives includeantioxidants, flame retardants, reinforcing agents such as glass fiber,asbestos fiber and flake, mineral fillers, stabilizers, nucleatingagents, ultraviolet light stabilizers, heat and light stabilizers,lubricants, dyes, pigments, toners, mold release agents, fillers, suchas glass beads and talc, and the like. Minor amounts of one or moreadditional polymers (e.g., up to about 10 percent by weight) optionallycan be incorporated in the present composition, such as polyamides,polycarbonates, polyethylenes, and polypropylenes. Antioxidants, thermalstabilizers, fillers, pigments and flame retardant additives, when used,preferably do not exert any adverse effect on impact strength.

[0040] The additive component of the thermoplastic compositionpreferably comprises a co-polymer of an ethylene monomer and aco-monomer that forms a polar moiety such as an acrylate co-monomer. Theadditive imparts toughness to the thermoplastic composition and makesthe composition particularly resistant to thermal treatments whichtraditionally result in toughness reduction. The polar or semi-polarnature of the additive also improves dispersion and mixing. Examples ofsuitable co-monomers include acrylates such as methylacrylate,butylacrylate, ethylacrylate, ethylhexyl methacrylate, and mixturesthereof. The concentration of the co-monomer should be between (a) aminimum which depends upon the identity of the acrylate and (b) anamount slightly less than the amount that makes the co-polymer amorphousor substantially amorphous. For example, when methylacrylate is used,its concentration preferably is from about 20 wt % to about 35 wt %,based on the total weight of the ethylene/methylacrylate co-polymer.Typical preferred acrylate concentrations range from about 7 wt % toabout 40 wt % and more typically from about 17 wt % to about 35 wt %,based on the total weight of the co-polymer. The average molecularweight of the co-polymer typically ranges from about 50,000 to about120,000. The melt flow index of the additive preferably is less thanabout 7, more preferably is less than about 3, and even more preferablyis less than about 2 g/10 min. The additive preferably has a relativelylow melting point and is thermally stable, e.g., does not degrade duringextrusion of the thermoplastic composition or during redrying of thethermoplastic composition in air for extended times.

[0041] The concentration of the additive component in the thermoplasticcomposition may be suitably selected according to properties requiredfor desired end uses of the composition. Typically, the concentration ofthe additive is from about 4 wt % to about 40 wt %, more typically fromabout 4 wt % to about 30 wt %, and even more typically from about 6 wt %to about 15 wt %, based on the total weight of the composition.

[0042] The additive co-polymer preferably has a major portion ofethylene, typically at least about 60 wt % and preferably at least about70 wt %, based on the total weight of the additive. The co-polymer alsomay contain one or more alpha-olefins having 3 to 10 or more carbonatoms. Illustrative examples include propylene, butene-1,pentene-1,3-methylbutene-1, hexene-1, octene-1, decene-1,4,4-dimethylpentene-1, 4,4-diethyl-hexene-1, 3,4-dimethylhexene-1,4-butyl-1-octene, 5-ethyl-i-decene, 3,3-dimethylbutene-1, mixturesthereof and the like. The preferred ethylene co-polymer comprises up toabout 5 wt % of other alpha-olefins as described above. In addition, theadditive optionally contains a core-shell toughener. Examples ofcore-shell tougheners which can be used are described in U.S. Pat. No.5,409,967, the disclosure of which is incorporated by reference hereinin its entirety.

[0043] The compatibilizer/emulsifier/surfactant (CES) preferably is agrafted or backbone-based co-polymer or ter-polymer comprising ethyleneand a glycidyl acrylate, such as glycidyl methacrylate or maleicanhydride. The CES co-polymer or ter-polymer preferably also includesother acrylates such as methylacrylate, ethylacrylate, propylacrylate,butylacrylate, ethylhexylacrylate, etc. Suitable exemplary amounts ofglycidyl acrylate or maleic anhydride range from about 0.1 wt % to about12 wt %, typically from about 1 wt % to about 10 wt %, and moretypically from about 2 wt % to about 8 wt %, based on the total weightof the co-polymer or ter-polymer. A grafted co-polymer or ter-polymertypically will have less glycidyl acrylate or maleic anhydride (e.g.,from about 0.2 wt % to about 1.5 wt %), whereas a backbone-basedco-polymer or ter-polymer can have higher amounts of glycidyl acrylateor maleic anhydride, e.g., as indicated above. Suitable exemplaryamounts of acrylate range from 0 to about 40 wt %, preferably from about10 wt % to about 30 wt %, and even more preferably from about 20 wt % toabout 35 wt %, based on the total weight of the co-polymer orter-polymer.

[0044] Typically, the concentration of the CES in the thermoplasticcomposition is from about 0.1 wt % to about 8 wt %, more typically isfrom about 0.2 wt % to about 6 wt %, and even more typically is fromabout 0.4 wt % to about 4 wt %. The melt flow index of the CESpreferably is less than about 20, more preferably is less than about 10,and even more preferably is less than about 6 g/10 min.

[0045] In one preferred embodiment of the present invention, the CEScomprises a ter-polymer of ethylene with about 8 wt % glycidylmethacrylate and about 25 wt % methylacrylate or butylacrylate, based onthe total weight of the ter-polymer. In another preferred embodiment,the CES comprises a co-polymer of ethylene with about 6 wt % glycidylmethacrylate, based on the total weight of the co-polymer. In anotherpreferred embodiment, the CES comprises a ter-polymer of ethylene withabout 2 wt % glycidyl methacrylate and from about 17 to 25 wt %methylacrylate, based on the total weight of the ter-polymer. In yetanother preferred embodiment, the CES comprises a ter-polymer ofethylene with about 3 wt % maleic anhydride and about 17 wt %butylacrylate, based on the total weight of the ter-polymer.

[0046] A cellular sheet can be made utilizing the thermoplasticcomposition of the present invention. Such cellular sheeting can be madeby mixing at least one inert gas with molten thermoplastic resincomposition in an extruder. This is done by simply injecting the inertgas into the molten resin in the extruder which is equipped with a sheetforming die. The inert gas used in this process can be any gas whichdoes not chemically react with the thermoplastic resin composition atthe elevated processing temperatures required. Some representativeexamples of inert gases which can be used include nitrogen, carbondioxide, helium, neon, argon, and krypton. For purposes of cost savingsand solubilities, nitrogen, carbon dioxide, or mixtures thereof normallywill be used as the inert gas.

[0047] A cellular sheet can be made with either a plasticating extruderor a melt extruder. Screw extruders of these type push the moltenthermoplastic resin composition containing discrete cells of the inertgas through a metal die that continuously shapes the sheet into thedesired form. In most cases, single screw extruders will be utilized.However, in some cases it may be desirable to utilize twin screwextruders or multiple screw extruders which perform essentially the samefunction.

[0048] In many cases it will be convenient to employ a plasticatingextruder of the single screw design. The thermoplastic resin compositionis fed into such a plasticating extruder by gravitational flow from ahopper into the screw channel. The thermoplastic resin composition fedinto the plasticating extruder is initially in particulate solid form.The thermoplastic resin composition initially enters the solid conveyingzone of the plasticating extruder. In the solid conveying zone, thesolid resin is conveyed and compressed by a drag-induced mechanism. Inthe solid conveying zone, the resin is mixed, heated, and conveyedthrough the extruder toward the melting zone. This heating is providedby maintaining the barrel of the extruder at an elevated temperature.The barrel of the extruder is typically heated electrically or by afluid heat exchanger system. Thermocouples are also normally placed inthe metal barrel wall to record and to help control barrel temperaturesettings.

[0049] Melting occurs in the melting zone after the resin is heated to atemperature which is above its melting point. In the melting zone,melting, pumping and mixing simultaneously occur. The molten resin isconveyed from the melting zone to the melt conveying zone. The inert gasis injected into the molten resin in the melt conveying zone. In themelt conveying zone, pumping and mixing simultaneously occur. The moltenresin in the melt conveying zone is maintained at a temperature which iswell above its melting point. A sufficient amount of agitation isprovided so as to result in an essentially homogeneous dispersion ofinert gas bubbles throughout the molten resin. The molten resin enteringthe melt conveying zone from the melting zone is at a somewhat lowertemperature and accordingly is of a higher viscosity. This essentiallyprevents the inert gas from back mixing through the extruder andescaping from the solid conveying zone via the hopper.

[0050] The molten thermoplastic resin composition in the melt conveyingzone typically is pumped into a metering pump and finally extrudedthrough a sheet- forming die. The metering pump and sheeting die aretypically maintained at a lower temperature than that of the barrelsurrounding the melt conveying zone to minimize rupture and diffusion ofinert gas bubbles in the thermoplastic resin composition. The sheetingdie is of a generally rectangular design which is quite wide and of asmall opening. Upon exiting the sheeting die, the sheet extrudate willswell to a level which is dependent upon the melt temperature, the dielength-to-opening ratio, and the shear stress at the die walls. In somecases, such as in the manufacture of clam shells, it is desirable to usea circular die and to extrude a tube which can be slit open andthermoformed. The cellular sheet produced typically is cooled withoutstretching by convected cold air or an inert gas, by immersion into afluid bath, or by passage over chilled rolls. The cellular sheetproduced is generally amorphous in nature.

[0051] The cellular sheet typically will contain a sufficient amount ofinert gas cells to provide it with a density which is within the rangeof about 0.4 to about 1.25. In most cases, the cellular sheet willcontain a quantity of inert gas cells to provide it with a density whichis within the range of 0.7 to 1.1. It generally is preferred for thecellular sheet to have a density which is within the range of about 0.8to about 1.0.

[0052] The cellular sheet can be thermoformed into heat-set, thin walledarticles utilizing conventional thermoforming equipment. Suchthermoforming typically is done by (1) preheating the substantiallyamorphous cellular sheet until it softens and positioning it over themold; (2) drawing the preheated sheet onto the heated mold surface; (3)heat-setting the formed sheet by maintaining sheet contact against theheated mold for a sufficient time period to partially crystallize thesheet; and (4) removing the part out of the mold cavity. In currentlyavailable thermoforming processes, the level of crystallinity of thepreformed sheet should not exceed about 10%.

[0053] The preheating of the substantially amorphous, cellular sheetprior to positioning over the thermoforming mold is necessary in orderto achieve the very short molding times required for a viable commercialprocess. The sheet must be heated above its T_(g) and below the point atwhich it sags excessively during positioning over the mold cavity. Inthe thermoforming process, a sheet temperature which is within the rangeof about 130° C. to about 210° C. and a mold temperature which is withinthe range of about 140° C. to about 220° C. will normally be utilized.It is often preferred to use a sheet temperature which is within therange of about 155° C. to about 185° C. and a mold temperature which iswithin the range of about 165° C. to about 195° C.

[0054] The invention can be practiced by using any of the knownthermoforming methods including vacuum assist, air assist, mechanicalplug assist or matched mold. The mold should be preheated to atemperature sufficient to achieve the degree of crystallinity desired.Selection of the optimum mold temperature is dependent upon the type ofthermoforming equipment, configuration and wall thickness of the articlebeing molded and other factors.

[0055] Heat-setting is a term describing the process of thermallyinducing crystallization of a polyester article in a restrainedposition. In the practice of the invention, heat-setting can be achievedby maintaining intimate contact of the solid or cellular sheet with theheated mold surface for a sufficient time to achieve a level ofcrystallinity which gives adequate physical properties to the finishedpart. For containers to be used in high temperature food application, alevel of crystallinity above 15% is preferable for adequate dimensionalstability during demolding operations, and more preferably is aboveabout 20% to yield parts with excellent dimensional stability and impactresistance.

[0056] The heat-set part can be removed from the mold cavity by knownmeans. One method, blow back, involves breaking the vacuum establishedbetween the mold and the formed sheet by the introduction of compressedair. In commercial thermoforming operation, the part is subsequentlytrimmed and the scrap ground and recycled.

[0057] Since a partially-crystalline finished article is necessary forgood dimensional stability at high temperatures, knowledge of the degreeof crystallinity or percent of crystallinity is of considerableimportance. The crystallinity of the polymer in such articles willnormally be measured by Differential Scanning Calorimetry (DSC). Theterms crystallization temperature and crystallization onset are usedinterchangeably to mean the temperature or temperature range in which aregularly repeating morphology, brought about by a combination ofmolecular mobility and secondary bonding forces, is induced in a polymerover a molecular distance of at least several hundred angstroms. Thecrystallization temperature or crystallization onset can be visuallyobserved as the point at which a substantially amorphous, non-orientedsheet of polymer changes from a translucent, hazy appearance to a whiteappearance.

[0058] As used throughout this specification and the appended claims,the term glass transition temperature (T_(g)) means that temperature ortemperature range at which a change in slope appears in the volumeversus temperature curve for said polymer and defining a temperatureregion below which the polymer exhibits a glassy characteristic andabove which the polymer exhibits a rubbery characteristic. The glasstransition temperature of polyethylene terephthalate is about 70° C.

[0059] According to another aspect of the invention, a multi-ply articlecan be produced by co-extrusion of two or more polymeric compositions.This technique may be used, e.g., for aesthetic purposes, such as inmaking a two-tone, ovenable container. If desired, two polymeric layerscan be co-extruded to “sandwich” a third layer. As will be appreciatedby those skilled in the art, a food-grade composition can be extrudedover a non-food grade composition to prepare an ovenable container.Preferred thermoplastic compositions of the present invention have goodsealability, e.g., permit packaging of refrigerated foods under pressureand the like. In some instances it may be desirable to extrude a moreamorphous layer over a highly crystalline layer, e.g., as in hermeticsealing. Such additional layers may be selected from a wide variety oforiented and non-oriented films of homo-polymers, co-polymers, andmixtures thereof which can be straight-chained, branched, or mixturesthereof. Examples of such polymers include polyesters such as PET, PEN,PETG, PCT, PCTA, PBT, PTT, and mixtures thereof. Suitable methods whichcan be used for co-extrusion are described in U.S. Pat. No. 4,533,510,4,929,482, and 5,318,811. A multi-ply article can have one or more solidlayers and/or one or more cellular layers, which can be sequenced in anydesired configuration.

[0060] A food-grade container can be prepared using a pre-driedthermoplastic composition containing polyethylene terephthalate (PET)having an I.V. of less than 0.95, 0.90, 0.85, or 0.80 (available fromShell Polyester), an additive, and a CES as described above. Thethermoplastic composition can be pre-dried, physically dry blended byweight, dried, melt mixed, devolitized, and processed through anextrusion die to form a sheet of desired thickness, using a single screwextruder, a twin screw extruder, or a multi-machine system(co-extruder). A multi-machine system can be used to apply a skin on oneor two sides to form a multi-ply article, e.g., to enhance sealing,aesthetics, gloss, color, and the like. For example, becausehigher-crystallinity films usually are more difficult to seal, it may beadvantageous to apply a second, more amorphous film, such as PETG or aniso-phthalic acid (IPA)/PET co-polymer, over a more crystalline firstlayer.

[0061] The single or multi-ply sheet can be fed over one or more sets ofshaping rolls. The shaping rolls cool the surface of the sheet toestablish a thermal gradient in the sheet while maintaining the plasticmass at a temperature suitable for vacuum forming. The rolls also can beused to enable laminations, to emboss the article, and the like. Theresult is a sheet of plastic having both a temperature gradientestablished therein and a bulk temperature at which vacuum forming canbe carried out.

[0062] The sheet then can be brought into contact with an assembly ofheated female molds and optional secondary cooling molds. Eithercontinuous, semi-continuous, or discontinuous processes can be used,which require different heat/cool/reheat sequences. The molds can bevented so that vacuum may be drawn in the mold. A vacuum is drawn withinthe mold, which draws the sheet into the mold to form an article in theshape of the mold. In this way, the sheet will be drawn into contactwith the temperature-controlled mold, and any air trapped between thesheet and the mold will be removed through the vent perforations.

[0063] The desired degree of crystallinity in regions of the formedarticle is different from that in other regions. Therefore, it ispreferred that while the thus-formed but still plastic article is incontact with the mold, regions of the mold are selectively heated so asto increase the rate of crystallization and/or to stress-release thepart. Region of the mold also can be selectively cooled to decrease therate of crystallization relative to other regions to achieve the desireddegree of crystallinity in each region. The various regions of thearticle in the mold are maintained at temperatures which yield anarticle having desired characteristics in the various regions defined bythe various temperature zones.

[0064] The various regions of the article in the mold typically aremaintained at temperatures sufficient to ensure that the bottom portionof the article has the highest degree of crystallization, i.e., is thewarmest portion; the top flange portion of the article is the leastcrystalline portion, i.e., is the coolest portion; and the remainingportions of the article between the bottom and the flange (i.e., thesides) are at intermediate temperature(s). In this way, the bottom andthe side have a higher degree of crystallization than does the flange ofthe formed article. The bottom thus has the highest heat resistance.

[0065] The article can be maintained in the mold at pre-selectedtemperatures for a time sufficient to form the article. Formed articlesmay be removed from the mold by removing the articles and the web ofplastic between them (the “trim”) from the forming apparatus as a unit,with the formed articles then separated from the trim and optionallyrelocated into a second mold at a specific temperature. Alternatively,the formed articles can be separated from the trim while the articlesare still in the molds. The trim is recovered separately from theindividual articles. The individual articles are ejected from the molds,and then are further processed. Without regard to which article/webseparation technique is used, at no time should the web be tensioned ineither direction, thus distorting the shaped product.

[0066] Surprisingly, the composition is particularly resistant tothermal treatments which traditionally cause toughness reduction due tocoarsening of the toughener phase and an increase in crystallinity ofthe matrix resin.

[0067] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the compositions and methodsof the present invention without departing from the spirit or scope ofthe invention. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

What is claimed is:
 1. A thermally crystallized thermoplastic polymeric composition having a degree of thermally induced crystallinity of at least about 15%, said composition comprising: a bulk polymer selected from the group consisting of PET, PEN, PETG, PCT, PCTA, PTT, and mixtures thereof, said bulk polymer optionally comprising up to about 10 wt % of a polyethylene based on the total weight of the bulk polymer; an additive in a concentration from about 4 wt % to about 40 wt %, based on a total weight of the composition, comprising a substantially amorphous co-polymer of ethylene and an acrylate; and a compatibilizer/emulsifier/surfactant (CES) in a concentration from about 0.1 wt % to about 8 wt %, based on the total weight of the composition, comprising a grafted or backbone co-polymer or ter-polymer of ethylene and a glycidyl acrylate or maleic anhydride, and optionally an acrylate selected from the group consisting of methylacrylate, ethylacrylate, propylacrylate, butylacrylate, ethylhexylmethacrylate, and mixtures thereof.
 2. The thermoplastic polymeric composition of claim 1 wherein said bulk polymer comprises one or more linear or branched homo-polymers, co-polymers, recycled polyesters, or a mixture thereof.
 3. The thermoplastic polymeric composition of claim 1 wherein said additive is selected from the group consisting of ethylene/methylacrylate co-polymer, ethylene/butylacrylate co-polymer, ethylene/ethylacrylate co-polymer, ethylene/ ethylhexylmethacrylate co-polymer, and mixtures thereof, and optionally contains a coreshell toughener.
 4. The thermoplastic polymeric composition of claim 3 wherein said additive co-polymer comprises from about 7 wt % to about 40 wt % of said acrylate, based on a total weight of said co-polymer.
 5. The thermoplastic polymeric composition of claim 4 wherein said additive copolymer comprises from about 17 wt % to about 35 wt % of said acrylate, based on the total weight of said co-polymer.
 6. The thermoplastic polymeric composition of claim 1 wherein the concentration of said additive is from about 4 wt % to about 30 wt %, based on the total weight of the composition.
 7. The thermoplastic polymeric composition of claim 6 wherein the concentration of said additive is from about 6 wt % to about 15 wt %.
 8. The thermoplastic polymeric composition of claim 1 wherein said CES is selected from the group consisting of ethylene/glycidyl methacrylate co-polymer, ethylene/maleic anhydride co-polymer, ethylene/glycidyl methacrylate/methylacrylate ter-polymer, ethylene/ glycidyl methacrylate/ethylacrylate ter-polymer, ethylene/glycidyl methacrylate/butylacrylate ter-polymer, ethylene/glycidyl methacrylate/ethylhexyl acrylate ter-polymer, ethylene/maleic anhydride/methylacrylate ter-polymer, ethylene/maleic anhydride/ethylacrylate ter-polymer, ethylene/maleic anhydride/butylacrylate ter-polymer, ethylene/maleic anhydride/ethylhexyl acrylate ter-polymer, and mixtures thereof.
 9. The thermoplastic polymeric composition of claim 8 wherein said CES concentration is from about 0.2 wt % to about 6 wt %, based on the total weight of the composition.
 10. The thermoplastic polymeric composition of claim 1 wherein said CES comprises a co-polymer or ter-polymer having from 0 to about 40 wt % of said acrylate and from about 0.1 to about 12 wt % of said glycidyl acrylate or maleic anhydride, based on a total weight of the co-polymer or ter-polymer.
 11. The thermoplastic polymeric composition of claim 10 wherein said CES comprises a ter-polymer having from about 10 wt % to about 30 wt % of said acrylate.
 12. The thermoplastic polymeric composition of claim 10 wherein said CES copolymer or ter-polymer has from about 1 wt % to about 10 wt % of said glycidyl acrylate or maleic anhydride.
 13. The thermoplastic polymeric composition of claim 1 wherein said bulk polymer comprises a blend of virgin and recycled polyesters.
 14. The thermoplastic polymeric composition of claim 1 wherein said bulk polymer comprises a blend of at least two polyesters having different intrinsic viscosities.
 15. The thermoplastic polymeric composition of claim 1 wherein said bulk polymer comprises at least one polyester having an intrinsic viscosity of less than about 0.95.
 16. The thermoplastic polymeric composition of claim 15 wherein said intrinsic viscosity is less than about 0.90.
 17. The thermoplastic polymeric composition of claim 16 wherein said intrinsic viscosity is less than about 0.85.
 18. The thermoplastic polymeric composition of claim 17 wherein said intrinsic viscosity is less than about 0.80.
 19. The thermoplastic polymeric composition of claim 18 wherein said intrinsic viscosity is about 0.5.
 20. A food-grade thermoplastic polymeric composition having a degree of thermally induced crystallinity of at least about 15%, said composition comprising: a bulk polymer selected from the group consisting of PET, PEN, PETG, PCT, PCTA, PTT, and mixtures thereof, said bulk polymer optionally comprising up to about 10 wt % of a polyethylene based on the total weight of the bulk polymer; an additive in a concentration from about 4 wt % to about 15 wt %, based on a total weight of the composition, comprising a substantially amorphous co-polymer of ethylene and an acrylate; and a compatibilizer/emulsifier/surfactant (CES) in a concentration from about 0.1 wt % to less than 4 wt %, based on the total weight of the composition, comprising a grafted or backbone co-polymer or ter-polymer of ethylene and a glycidyl acrylate or maleic anhydride, and optionally an acrylate selected from the group consisting of methylacrylate, ethylacrylate, propylacrylate, butylacrylate, ethylhexylacrylate, and mixtures thereof.
 21. A layered thermoplastic polymeric composition comprising: (a) a first thermoplastic polymeric layer having a degree of thermally induced crystallinity of at least about 15%, said first layer comprising: (i) a bulk polymer selected from the group consisting of PET, PEN, PETG, PCT, PCTA, PTT, and mixtures thereof, said bulk polymer optionally comprising up to about 10 wt % of a polyethylene based on the total weight of the bulk polymer; (ii) an additive in a concentration from about 4 wt % to about 40 wt %, based on a total weight of the composition, comprising a substantially amorphous co-polymer of ethylene and an acrylate; and (iii) a compatibilizer/emulsifier/surfactant (CES) in a concentration from about 0.1 wt % to about 8 wt %, based on the total weight of the composition, comprising a grafted or backbone co-polymer or ter-polymer of ethylene and a glycidyl acrylate or maleic anhydride, and optionally an acrylate selected from the group consisting of methylacrylate, ethylacrylate, propylacrylate, butylacrylate, ethylhexyl methacrylate, and mixtures thereof; and (b) a second polymeric layer laminated to or co-extruded onto the first layer.
 22. The layered thermoplastic polymeric composition of claim 21 wherein said second polymeric layer is selected from the group consisting of PET, PEN, PETG, PCT, PCTA, PBT, PTT, and mixtures thereof. 